This invention relates to a monitor for determining the oxygen concentration in a reactive metal film deposition, particularly, the oxygen content in an aluminum film deposited in a vacuum chamber containing residual oxygen species such as water vapor and/or added oxygen.
The properties of aluminum integrated-circuit metallizations are profoundly affected by impurities which are incorporated in the aluminum during film deposition. Elements, such as copper and oxygen, have been deliberately incorporated in aluminum films to provide increased resistance to electromigration failure in circuits operating at high-current densities. Silicon has been added to aluminum to inhibit junction spiking during the contact alloying process, and it has recently been shown that periodic addition of oxygen to aluminum during film deposition eliminates the growth of hillocks which otherwise occurs during the alloying process. See my copening patent application, Ser. No. 201,438, filed on Oct. 28, 1980, now U.S. Pat. No. 4,302,498, entitled "LAMINATED CONDUCTING FILM ON AN INTEGRATED CIRCUIT SUBSTRATE AND METHOD OF FORMING THE LAMINATE" for a detailed description of this process.
Oxygen is ubiquitous in aluminum integrated-circuit films. In the absence of a deliberate oxygen input, the principal source of oxygen is residual H.sub.2 O in the vacuum system at the start of film deposition. Residual gas pressure in the vacuum chamber during deposition is the 10.sup.-6 Torr range. Typically, over 90 percent of this is water vapor outgassing from the chamber walls. A fraction of the water vapor molecules which impinge on the substrate react with the growing aluminum film. Hydrogen is usually released in such reactions, but the oxygen is adsorbed and incorporated in the film. Typical oxygen concentrations in a film deposited with no deliberate oxygen inputs are on the order of 0.1 percent. However, wide variations in concentration can occur in climates where variations in relative humidity are experienced, and/or where throughput considerations cause vacuum-system pumpdown times and maintenance schedules to be kept at a minimum.
It is known that oxygen (1) increases aluminum film hardness and resistivity; (2) decreases as-deposited film stress; (3) increases resistance to electromigration; (4) decreases grain size; and (5) eliminates hillock growth, given the proper oxygen-input sequencing conditions. In spite of these important influences on film properties, there are no methods for monitoring oxygen concentrations in aluminum films which are useful in terms of being readily applied to factory situations. Residual gas analysis involves expensive, sophisticated equipment, and the relationship between the data obtained and oxygen concentration in the film is tenuous. Materials analysis techniques such as Secondary Ion Mass Spectrometry (SIMS) also involve expensive equipment and cannot provide results immediately after the completion of film deposition.
A simple method and apparatus for determining oxygen concentration would therefore provide a valuable process-control monitor for films deposited in the residual vacuum-chamber environment, and also provide an important tool in the development of films with deliberate oxygen additions.